Proteinaceous materials are used for a wide variety of applications. One of the predominant useful properties is their ability to dissolve in aqueous solutions and yet form a solid matrix which is permeable to aqueous solutions upon drying. These properties have been exploited for many generations in the field of photographic sciences and proteinaceous materials are still widely used as a binder for harboring silver halide grains in the photosensitive layer of most photographic films.
Formation of a solid matrix is typically considered to be a result of inter-and intra-molecular hydrogen bonding within both the crystalline and amorphous regions of proteinaceous materials. If only the natural hydrogen bonding is employed, the strength of the wetted matrix is typically insufficient for use in a photographic film. Therefore, it is common practice to add a crosslinking agent, also known as a hardener, to a protein material when used for photographic layers.
Hardeners are chosen, in part, for their ability to link one group on a protein molecule with another group on the same, or different, protein molecule. The linking generates a three-dimensional network of proteinaceous material. This three-dimensional network has sufficient integrity to allow swell during processing without causing detrimental effects to the silver halide grain harbored therein. Another important aspect of the three-dimensional network is an ability to allow solution to permeate freely during the photographic processing steps of development, fix (or bleach) and wash. It is also imperative that the aqueous solution which freely permeates the matrix is not strongly absorbed. A matrix with high affinity to water, or other solutions is more difficult to dry. This is particularly important for photosensitive elements since they must often be capable of transiting the photographic processing steps of development, fix, wash and dry in 20-120 sec. Carboxyl groups can be the dominant hydrophilic species on a proteinaceous molecule. Carboxyl groups are either pendant to the proteinaceous molecule, or integral to a side chain. It is therefore desirable to alter the carboxyl groups in some way to decrease the water affinity of the matrix.
Peptide couplers are known in the art to couple two regions of hydrophilic colloid thereby creating the aforementioned three-dimensional network as a compilation of interconnected strands of hydrophilic colloid. A particular advantage of peptide couplers is their ability to couple a carboxyl group with an amine group to form an amide bond between two portions of the proteinaceous material. This has many benefits including increasing the strength of the cross-linked matrix and decreasing the number of..unreacted carboxyl groups and thus the overall water affinity of the matrix.
In addition to crosslinking the proteinaceous material, it is often desirable to further augment this hydrophilic colloid network with other ingredients such as hydrophilic or hydrophobic groups, dyes, plasticizers and the like. Previously, the augmentation has taken one of several forms. Either an adjuvant is admixed with the hydrophilic colloid or the gelatin is derivatized to include pendant groups which can impart specific properties to the gelatin. Admixed ingredients can typically leach out during any wet procedure and can also diffuse through the matrix structure decrease their effectiveness. Many times the admixed ingredients are not permanent. Derivatizing is detailed in EP 576,912A and EP 576,911A. A derivatization reaction typically utilizes a crosslinking site and therefore decreases the number of carboxyl or amine groups available for crosslinking. A decrease in the number available crosslinking sites could affect the amount of crosslinked matrix and subsequently decrease the strength of the binder.
There has been a long felt need in the art to provide a hardener which can effectively incorporate additives to the hydrophilic colloid layer while concurrently crosslinking the strands of hydrophilic colloid. There has been a particular need to incorporate additives as a bridge between two strands of hydrophilic colloid. These bridge groups can be dual purpose since they act as crosslinkers and may impart other properties to the hydrophilic colloid layer.